Abstract

Measurements were made of the formative time 1ag in helium and an analysis due to Davidson was used to obtain the relative contributions of the individual secondary processes to the current growth.A series of curves was obtained, giving the dependence of the formative time lag on the percentage overvo1tage, for overvo1tages within 4% of the sparking potential. These curves were obtained at five pressures in the range 14.9 to 62.2 torr and at gap distances from 0.18 to 0.82 cm. The range of E /p was from 9.8 to 38.7 volt cm-1 torr-1.The combination of two processes involving delayed photons gave good agreement with experiment over the intermediate range of E /p investigated, one of the delayed photon processes was the emission of secondary electrons from the cathode by photons produced by the decay of 21S metastable atoms in two-body collisions in the gas. The other was due to photons from the decay of 23 metastable molecules which had been produced in three-body collisions involving 23s metastable atoms.Positive ion action at the cathode became important at the highest values of E /p used. This, in combination with the action of delayed photons from the decay of 21s metastable atoms, gave the required agreement with experiment.Below 11. 6 volt cm-1 torr-1, the observed time lags could not be satisfactorily explained by the processes considered. In this case, it was suggested that examination of the processes involving the action at the cathode of metastab1e atoms and delayed photons from the decay of metastab1e molecules might prove fruitful.